Photoresponsive Organic Molecular Crystals

Dear Colleagues,

The symmetry, faceted nature, and vibrant colours of organic crystals are unparalleled in beauty. In the hands of a capable crystallographer, these crystals can reveal a hidden treasure trove of information relating to their structure and composition. These crystals are considered too brittle and fragile for practical applications. However, the need to manufacture thinner, flexible, and wearable electronic devices has shifted attention towards organic molecules that show unique properties in the condensed or crystalline state. Recently, researchers discovered that micron-sized crystals can have elastic or plastic properties and can even be used as active waveguides capable of transmitting coded information. Of special interest is the emergence of the so-called photomechanical or photoresponsive molecular crystals. These crystals are composed of photoresponsive molecules (chromophores) that can convert light to chemical energy via bond breaking/bond forming. When the chromophores are aligned in a favourable geometry (crystal engineering), stimulating these crystals with light leads to the amplification of the invisible Angstrom (Å) level molecular motion to an observable effect that perturbs the shape of the entire crystal. This perturbation or photomechanical response can take on many forms depending on the chemical composition, size, shape, and crystal packing. In many cases, a photosalient response is observed wherein the crystal shatters violently to relieve built-up stress between the reactant and photoproduct phase. Through clever engineering of the crystal habit, size, functional groups, and photochemistry, we can make photomechanical crystals that relieve photochemical stress build-up by bending, twisting, curling, peeling, jumping, crawling, or wiggling, etc. When the crystal is made up of a photoisomerizable chromophore such as diaryl ethenes, azobenzenes or anthracene diolefins crystals can function as light-activated actuators, toggling between different shapes as a function of the wavelength and direction of light used. Photomechanical crystals have been steadily gaining momentum over the past decade with new mechanisms being investigated to help improve the quantum yield, work output, and response speed.

We invite scientists and researchers to contribute to this Special Issue of Crystals entitled “Photoresponsive Organic Molecular Crystals”. This Issue aims to build a contemporary collection of recent pioneering work and advances in the field of photoresponsive molecular crystals, involving molecular structure design, crystal engineering, material fabrication, and potential applications. We hope to shed light on the potential use of these “smart” crystals in real-world applications by incorporating material engineering and machine learning.

Prof. Dr. Fei Tong
Prof. Dr. Rabih O. Al-Kaysi
Prof. Dr. Daichi Kitagawa
Guest Editors

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